1. The Field of the Invention
The present invention relates in general to the fabrication of a photodiode to achieve fiber modal compensation in optical fiber systems.
2. Description of the Related Art
In an optical communication system that includes a multi-mode fiber (MMF), an optical signal launched into a fiber propagates along the fiber in multiple mode groups, each of which exhibits a different group velocity. Portions of the source optical signal reside in the different mode groups. Each of the mode groups can have different arrival times at the end of the fiber due to the different group velocities. The different group velocities of the mode groups cause a pulse, representing a binary symbol, formed from more than one mode group to spread out as it propagates. This is because portions of the symbol travel faster than other portions of the symbol. This is referred to as modal dispersion. Modal dispersion causes the optical signal initially launched through a fiber at a predetermined frequency and an initial phase to vary as a function of the length of the fiber.
Modal dispersion reduces the maximum data transmission rate of the optical communication system and thus diminishes the total transmission capacity of the fiber. This results at least in part from the fact that modal dispersion spreads the optical pulse as it propagates. Adjacent pulses may spread into each other causing intersymbol interference. Short pulses typically also include short times between pulses and thus are limited to very short transmission distances to prevent or minimize intersymbol interference. Longer pulses can be transmitted further because the relative distortion of the pulse is smaller and distance between pulses is greater. Shorter pulses typically require more bandwidth for propagation. A common specification criteria for multimode fiber is a bandwidth-distance product. Importantly, the bandwidth-distance products of typical multimode fiber are severely limiting. Typically the fiber bandwidth is specified using a light launching condition that fills all of the modes of the optical fiber.
Modem multimode fiber incorporates a graded optical index profile within the core of the fiber to reduce modal dispersion. This is done by engineering the index profile of the graded region, typically in the shape of a parabola along the transverse dimension of the optical fiber. However, manufacturing variances and defects such as uncontrolled changes in grading profiles, a so called “alpha change”, or index peaking or dipping in the center, lead to variances in the fiber modal bandwidth. Modal dispersion remains the dominant bandwidth limiting mechanism in multimode fibers. These deviations can lead to significant variations in the optical bandwidth of the fiber depending on which mode groups are excited in the launching condition. Furthermore, as compared to single mode fibers, these limits reduce the capacity of multimode fiber by orders of magnitude.
The different propagation velocities of the distinct optical mode groups can lead to large differential mode delay (DMD) in multimode fibers. The severity of this modal dispersion can limit multimode fiber links to bandwidth-distance products of a few tens of MHz-kilometers depending on the original mode excitations.
The mode groups inside the fiber are characterized with different divergences at the distal end of the optical fiber. Specifically, higher order mode groups have a wider divergence angle than lower order mode groups. Stated differently, when higher order mode groups exit a multimode fiber onto a detector, such as a photodiode, the higher order modes will impinge the detector within a larger radius than the lower order modes.
Therefore, it is possible to spatially separate the various mode groups in the optical fiber, and by appropriate reconstruction and delays, correct the effects of modal dispersion in the fiber.
In practice it is often sufficient to only spatially resolve two areas, the inner and outer portion. Electronic compensation of the fiber modal dispersion can then be used to restore the signal integrity. This has been demonstrated to improve the bandwidth of the optical system. In previously described systems, a photodiode has been fabricated with two concentric active regions. The response of the two active regions can be subtracted electronically to reduce the effects of the modal dispersion. The approach therein is to use a Metal-Semiconductor-Metal (MSM) photodiode with two active areas. However, this has the disadvantage of requiring complex fabrication technology, and external subtraction circuits to operate effectively.
Thus, a need exists in the industry to address the aforementioned and/or other deficiencies and/or inadequacies.